Dye diffusion thermal transfer printing

ABSTRACT

A thermal transfer printing dye sheet comprising a substrate, a dye coat comprising a dye and a polymeric binder on the substrate and an over coat of styrene/butadiene copolymer on the dye coat is disclosed.

INTRODUCTION

This invention relates to dye diffusion thermal transfer printing (DDTTPor D2T2 printing, D2T2 is a trade mark of Imperial Chemical IndustriesPLC)

It is known to print woven or knitted textile material by a thermaltransfer printing (TTP) process. In such a process a sublimable dye isapplied to a paper substrate (usually as an ink also containing aresinous or polymeric binder to bind the dye to the substrate until itis required for printing) in the form of a pattern, to produce atransfer sheet comprising a paper substrate printed with a pattern whichit is desired to transfer to the textile. Substantially all the dye isthen transferred from the transfer sheet to the textile material, toform an identical pattern on the textile material, by placing thepatterned side of the transfer sheet in contact with the textilematerial and heating the sandwich, under light pressure from a heatedplate, to a temperature from 180-220° C., for a period of 30-120seconds.

As the surface of the textile substrate is fibrous and uneven it willnot be in contact with the printed pattern on the transfer sheet overthe whole of the pattern area. It is therefore necessary for the dye tobe sublimable and vaporise during passage from the transfer sheet to thetextile substrate in order for dye to be transferred from the transfersheet to the textile substrate over the whole of the pattern area.

As heat is applied evenly over the whole area of the sandwich over asufficiently long period for equilibrium to be established, conditionsare substantially isothermal, the process is non-selective and the dyepenetrates deeply into the fibres of the textile material.

In DDTTP, a dye sheet is formed by applying a heat-transferable dye(usually in the form of a solution or dispersion in a liquid alsocontaining a polymeric or resinous binder to bind the dye to thesubstrate) to a thin (usually <20 micron) substrate having a smoothplain surface in the form of a continuous even film over the entireprinting area of the dye sheet. Dye is then selectively transferred fromthe transfer sheet by placing it in contact with a material having asmooth surface with an affinity for the dye, hereinafter called thereceiver sheet, and selectively heating discrete areas of the reverseside of the dye sheet for periods from about 1 to 20 milliseconds (msec)and temperatures up to 300° C., in accordance with a pattern informationsignal, whereby dye from the selectively heated regions of the dye sheetdiffuses from the dye sheet to the receiver sheet and forms a patternthereon in accordance with the pattern in which heat is applied to thedye sheet. The shape of the pattern is determined by the number andlocation of the discrete areas which are subjected to heating and thedepth of shade in any discrete area is determined by the period of timefor which it is heated and the temperature reached.

Heating is generally, though not necessarily, effected by a line ofheating elements, over which the receiver and transfer sheets are passedtogether. Each element is approximately square in overall shape,although the element may optionally be split down the centre, and may beresistively heated by an electrical current passed through it fromadjacent circuitry. Each element normally corresponds to an element ofimage information and can be separately heated to 300° C. to 400° C., inless than 20 msec and preferably less than 10 msec, usually by anelectric pulse in response to a pattern information signal. During theheating period the temperature of an element will rise to about 300-400°C. over about 5-8 msec. With increase in temperature and time more dyewill diffuse from the dye sheet to the receiver sheet and thus theamount of dye transferred onto, and the depth of shade at, any discretearea on the receiver sheet will depend on the period for which anelement is heated while it is in contact with the reverse side of thedye sheet.

As heat is applied through individually energised elements for veryshort periods of time the process is selective in terms of location andquantity of dye transferred and the transferred dye remains close to thesurface of the receiver sheet.

As an alternative heating may be effected using a light source in alight-induced thermal transfer (LITT or L2T2 printing, L2T2 is a trademark of Imperial Chemical Industries PLC) printer where the light sourcecan be focused, in response to an electronic pattern information signal,on each area of the dye sheet to be heated. The heat for effectingtransfer of the dye from the dye sheet is generated in the dye sheetwhich has an absorber for the inducing light. The absorber is selectedaccording to the light source used and converts the light to thermalenergy, at a point at which the light is incident, sufficient totransfer the dye at that point to the corresponding position on thereceiver sheet. The inducing light usually has a narrow waveband and maybe in the visible, infra-red or ultra violet regions although infra-redemitting lasers are particularly suitable.

In DDTTP it is important that the surfaces of the dye sheet and receiversheet are even so that good contact can be achieved between the printedsurface of the dye sheet and the receiving surface of the receiver sheetover the entire printing area because it is believed that the dye istransferred substantially by diffusion in the molten state in condensedphases. Thus, any defect or speck of dust which prevents good contactover any part of the printing area will inhibit transfer and lead to anunprinted portion on the receiver sheet on the area where good contactis prevented, which can be considerably larger than the area of thespeck or defect. The surfaces of the substrate of the dye and receiversheets are usually a smooth polymeric film, especially of a polyester,which has some affinity for the dye.

The temperature and pressure involved during the thermal transferprinting process produce ideal conditions for adhesion between thepolymeric materials forming the dye sheet and the receiver sheetpreventing clean separation of the dye sheet from the receiver sheetafter printing.

It is known to overcome this problem by incorporating suitable releasematerial into the receiver coat. Silicone-type materials areparticularly well known for this purpose. However, further problems thenarise as the receiver coat chemistry can change with ageing causingdifferences in product performance and the presence of the releasematerial at the surface of the receiver coat can affect adhesion of aprotective overlayer, eg in security laminates.

These latter problems may be overcome by providing the dye sheet withrelease properties, for example by overcoating the dye coat with a layerof hydroxypropymethyl cellulose as disclosed in GB-B-2157841.

Whilst this solution is satisfactory when the polymer of the receiversheet is polyester, when more hydrophilic polymers, such as poly (vinylpyridine) or poly vinyl pyrrolidone are used total transfer occurs. Theadhesion between the dye coat and the receiver coat is so strong thatfailure occurs at the interface between the dye coat and substrate withthe result that the complete dye coat, ie the dye and the polymertransfer.

According to one aspect of the present invention, there is provided adye sheet comprising a substrate, a dye coat comprising a dye and apolymeric binder on the substrate and an over coat of styrenelbutadienecopolymer on the dye coat.

According to a further aspect of the invention, there is provided athermal transfer printing dye sheet/receiver sheet combination in whichthe dye sheet comprises a substrate, a dye coat comprising a dye and apolymeric binder on the substrate and an over coat of styrene/butadienecopolymer on the dye coat and in which the receiver sheet comprises asubstrate having thereon a receiver coat consisting of a polymer morehydrophilic than polyester such as poly (vinyl pyridine), poly (vinylpyrrolidone) or a vinyl pyrrolidone/vinyl acetate copolymer.

The over coat may have a thickness of 0.01 to 0.5 μm, preferably 0.05 to0.25 μm.

The ratio of styrene to butadiene may be from 1:2 to 2:1

The over coat may be applied as a 0.5 to 3% w/w solution in a suitableorganic solvent such as hexane.

According to a preferred aspect of the invention, the dye has theformula ##STR1## in which Ch is a chromogen

Ra and Rb each independently is a spacer group

Y is an interactive functional group

w and x each independently is 0 or an integer equal to or greater than1, and

m and n each independently is an integer equal to or greater than 1,provided that w and x are not both equal to 0 and when one of w or x is0 at least one of m and n is equal to or greater than 2.

In this specification, the term "chromogen" is defined as meaning thearrangement of atoms which substantially governs the absorbance ofelectromagnetic radiation by the dye molecule and particularly in thecase of visible radiation, the arrangement of atoms which causes the dyemolecule to be coloured.

The spacer groups represented by R^(a) and R^(b) may be any groupscapable of carrying one or more interactive functional groups (Y) andminimising steric and electronic effects of the Y group and therebyminimising any changes in the absorption characteristics of thechromogen group Ch and thus shade which the Y group would otherwisecause.

Preferably the spacer groups each comprise an atom or group of atomsconnected to the chromagen by at least one sigma bond and to theinteractive group by at least one sigma bond.

The spacer groups may contain at least one of a carbon, silicon orsulphur atom, preferably two carbon atoms and more preferably from threeto ten carbon atoms.

The interactive functional group represented by Y are such that the Ygroups on different dye molecules may be interact with each other toform dye complexes of larger size and thus of lower mobility and/or theY groups may interact with a dye receptive polymer such as thosementioned above, on the receiver sheet. The Y groups may be the same ordifferent and the R^(a) and R^(b) may carry one more Y groups. Theinteractions between different Y groups or between the Y groups and thedye receptive polymer produces an image on the receiver sheet which isresistant to crystallisation and migration of the dyes is minimised. TheY groups are preferably selected from OH, NH₂, NHR, NR₂, COOH, CONH₂,NHCOR, CONHR, SO2NH₂, SO2NHR, SO₃ H, NHCONH₂, NHCONHR, ═NOH, and PO₃ H,in which R is selected from --CN, NO₂, --Cl, --F, --Br, C₁₋₆ alkyl, C₁₋₆alkoxy, --NHCOC₁₋₆ alkyl, --NHCOphenyl, --NHSO₂ alkyl NHSO₂ phenyl oraryloxy,

Preferred dyes for use in the dye coat are described in the followingexamples or are disclosed in co-pending PCT application claimingpriority from Applications Nos GB 9508810.0, 9508874.6 and GB 9508880.3

The Coating

The coating suitably comprises a binder together with a dye or mixtureof dyes. The ratio of binder to dye is preferably at least 0.7:1 andmore preferably from 1:1 to 4:1 and especially preferably 1:1 to 2:1 inorder to provide good adhesion between the dye and the substrate andinhibit migration of the dye during storage.

The coating may also contain other additives, such as curing agents,preservatives, etc., these and other ingredients being described morefully in EP 133011A, EP 133012A and EP 111004A.

The Binder

The binder may be any resinous or polymeric material suitable forbinding the dye to the substrate which has acceptable solubility in theink medium, i.e. the medium in which the dye and binder are applied tothe transfer sheet. It is preferred however, that the dye is soluble inthe binder so that it can exist as a solid solution in the binder on thetransfer sheet. In this form it is generally more resistant to migrationand crystallisation during storage. Examples of binders includecellulose derivatives, such as ethylhydroxyethylcellulose (EHEC),hydroxypropylcellulose (HPC), ethylcellulose, methylcellulose, celluloseacetate and cellulose acetate butyrate; carbohydrate derivatives, suchas starch; alginic acid derivatives; alkyd resins; vinyl resins andderivatives, such as polyvinyl alcohol, polyvinyl acetate, polyvinylbutyral, polyvinyl acetoacetal and polyvinyl pyrrolidone; polycarbonatessuch as AL-71 from Mitsubishi Gas Chemicals and MAKROLON 2040 from Bayer(MAKROLON is a trade mark); polymers and co-polymers derived fromacrylates and acrylate derivatives, such as polyacrylic acid, polymethylmethacrylate and styrene-acrylate copolymers, styrene derivatives suchas polystyrene, polyester resins, polyamide resins, such as melamines;polyurea and polyurethane resins; organosilicones, such aspolysiloxanes, epoxy resins and natural resins, such as gum tragacanthand gum arabic. Mixtures of two or more of the above resins may also beused, mixtures preferably comprise a vinyl resin or derivative and acellulose derivative, more preferably the mixture comprises polyvinylbutyral and ethylcellulose. It is also preferred to use a binder ormixture of binders which is soluble in one of the above-mentionedcommercially acceptable organic solvents.

The Substrate

The substrate may be any sheet material preferably having at least onesmooth even surface and capable of withstanding the temperaturesinvolved in DDTTP, i.e. up to 400° C. for periods up to 20 msec, yetthin enough to transmit heat applied on one side through to the dyes onthe other side to effect transfer of the dye onto a receiver sheetwithin such short periods. Examples of suitable materials are polymers,especially polyester, polyacrylate, polyamide, cellulosic andpolyalkylene films, metallised forms thereof, including co-polymer andlaminated films, especially laminates incorporating a smooth evenpolyester receptor layer on which the dye is deposited. Thin (<20micron) high quality paper of even thickness and having a smooth coatedsurface, such as capacitor paper, is also suitable. A laminatedsubstrate preferably comprises a backcoat, on the opposite side of thelaminate from the receptor layer, which, in the printing process, holdsthe molten mass together, such as a thermosetting resin, e.g a silicone,acrylate or polyurethane resin, to separate the heat source from thepolyester and prevent melting of the latter during the DDTTP operation.The thickness of the substrate depends to some extent upon its thermalconductivity but it is preferably less than 20 μm and more preferablyless than 10 μm.

The following non-limiting examples illustrate the invention.

EXAMPLE 1

Dye sheet samples were prepared by coating 6 μm thick polyethyleneterephthalate substrate (supplied by Diafoil) from a solution containing

    ______________________________________    dye                    2.1% w/w    poly(vinyl butyral)    2.1% w/w    tetrahydrofuran       95.8% w/w    ______________________________________

using a K2 wire bar, drying the resultant coating at 110° C. for 20seconds and overcoating the dried dye coat with a 1.2% w/w solution inhexane of a copolymer containing equal amounts of styrene and butadiene(Europrene Sol S141 from Enichem) using a K2 wire bar to give a finalrelease layer 0.15 μm thick.

Further samples were prepared in the same way with the omission of therelease layer for comparison purposes.

Receiver sheet samples were prepared by coating 140 μm polyethyleneterephthalate substrate (Melinex® 990 from ICI) with a solutioncontaining

    ______________________________________           polymer      11.1% w/w           solvent      89.9% w/w    ______________________________________

using a K4 wire bar and drying the resulting coating at 140° C. for 3minutes.

The dyes used were

1) an azopyridone as used in commercial dye sheets and having theformula ##STR2## 2) a dye having the formula ##STR3## where R1 and R2are aliphatic carboxyl groups and R3 is alkyl; 3) as dye 2 except thatR1, R2, and R3 are aliphatic hyroxyl groups;

4 a propyl analogue of azopyridone having a melting point of 288° C. andthe formula ##STR4##

The receiver sheet polymers used were

A) polyester (Vylon® 200 from Toyobo)

B) poly(vinyl pyridine)

C) poly (vinyl pyrrolidone)

D) vinyl pyrrolidone/vinyl acetate copolymer (VA64 from BASF)

Polymers A, B and C were coated from tetrahydrofuran and polymer D wascoated from water.

Each possible dye sheet/receiver sheet combination was printed using alaboratory thermal printer having a head voltage of 12V for a print timeof 18 ms. There was no problem of separation in the presence of therelease layer and a satisfactory image was obtained. In the absence ofthe release layer it was not possible to produce an image because ofhigh adhesion between the dye coat and the receiver coat. This led tointerfacial failure at the substrate/dye coat interface causing thewhole dye coat to transfer.

We claim:
 1. A thermal transfer printing dye sheet comprising asubstrate, a dye coat comprising a dye and a polymeric binder on thesubstrate and an over coat of styrene/butadiene copolymer on the dyecoat.
 2. A thermal transfer printing dye sheet according to claim 1, inwhich the over coat has a thickness of 0.01 to 0.5 μm.
 3. A thermaltransfer printing dye sheet according to claim 1, in which the dye hasthe formula ##STR5## in which Ch is a chromogen as herein definedRa andRb each independently is a spacer group Y is an interactive functionalgroup w and x each independently is 0 or an integer equal to or greaterthan 1, and m and n each independently is an integer equal to or greaterthan 1, provided that w and x are not both equal to 0 and when one of wor x is 0 at least one of m and n is equal to or greater than
 2. 4. Athermal transfer printing dye sheet/receiver sheet combination in whichthe dye sheet comprises a substrate, a dye coat comprising a dye and apolymeric binder on the substrate and an over coat of styrene/butadienecopolymer on the dye coat and in which the receiver sheet comprises asubstrate having thereon a receiver coat consisting of a polymer morehydrophilic than polyester.
 5. A thermal transfer printing dyesheet/receiver sheet combination according to claim 4, in which thereceiver coat polymer is poly (vinyl pyridine), poly (vinyl pyrrolidone)or a vinyl pyrrolidone/vinyl acetate copolymer.